What is the difference between gitlab-runner start and gitlab-runner run?

The documentation is not very clear on this:

gitlab-runner run

This is main command that is executed when GitLab Runner is started asa service. It reads all defined runners from config.toml and tries torun all of them.

The command is executed and works until it receives a signal.

gitlab-runner start

This command starts the GitLab Runner service.

Furthermore, I've found that installing and registering a runner and then starting it as a service is sufficient to get it working. What, then is the purpose of the run command?

Thanks